1.notident.. Field of the Invention
This invention relates to motor driven pumps of the canned or isolated stator type, with magnetic thrust balancing and independently controlled fluid flow for cooling.
2. Description of the Prior Art
Motor driven pummps having motor stators in which the stator electrical windings are isolated from the pumped fluids, are called "canned" motor pumps and are well known in the art. Such pumps are often utilized where hot, corrosive, toxic and expensive fluids are handled, and are most desired because they eliminate rotating seals and stuffing boxes which connect the hydraulic or pumping apparatus to a motor, which could leak fluids to the atmosphere and cause hazardous conditions and expense.
Such pumps are often in continuous use while handling hot fluids, which causes rapid heat build up and subsequent damage to the electrical components of the pumps.
All electric motors require cooling to properly perform, and most standard motors have built-in fans to provide cooling. A pumnp that has a canned motor stator which is hermetically sealed cannot be cooled in this fashion and, therefore, the practice has been to divert some of the fluid that is being handled so that it is recirculated through the motor betwen the rotor and canned stator where it draws the heat away. This design is exemplified in the U.S. Patents to White, U.S. Pat. No. 2,906,208; and to Litzenberg, U.S. Pat. Nos. 2,871,791 and 4,065,237.
Other designs use external fluid circulation, as illustrated in the U.S. Patents to White, U.S. Pat. Nos. 2,713,311; and 3,053,189; and 3,114,090.
Canned motor pumps that utilize internal fluid circulation depend upon the clearance between the rotor and stator as an orifice for controlling the amount of fluid flow through the motor section. The following difficulties arise from this method of fluid control:
1. The variances in the manufacturing tolerances which constitute this fluid gap are of such a magnitdue that either more or less of the desired flow is obtained; PA1 2. The rotor and stator of a canned motor pump vary in length depending upon the horsepower of the pump, and this results in an unstable and unpredictable volume of fluid; and PA1 3. The fluid pressure developed in the volute section is completely variable due to the operating conditions, and coupled with 1 and 2 above does not provide an accurate flow for the purpoase of cooling the motor.
Fluid flow for the purpose of stator cooling must vary with the horsepower developed by the motor, too much causes a drop in efficiency of the pump and too little will reduce the service life of the pump. For example, extensive tests have established that 3 GPM of room temperature fluid is required to properly cool the stator section in a 3 HP motor. Any more fluid is unnecessary and seriously affects the hydraulic efficiency of the unit, and any less may be detrimental to the service life of the motor windings. Tests on hundreds of production units have positively demonstrated, that due to the conditions stated above, the use of the clearance between the rotor and stator sections for fluid flow does not control the fluid flow in a satisfactory manner.
The utilization of an orifice of predeterminined size will provide the proper fluid flow control, but this orifice must be sized for the individual horsepower and motor cooling requirements of the pumps and is not adjustable. The fixed orifice is mounted in a plate that has no other openings of consequence from the pump chamber to the stator and rotor section. The orifice is a control orifice to permit the fluid flow to continue to the hollow shaft, which returns recirculated fluid to suction or to the impeller. While this orifice eliminates the deviations that arise from the use of the gap between the rotor and stator as a metering medium for cooling or other purposed, it does not properly control fluid flow for thrust balancing.
In addition with certain high temperature applications, the flow of fluid from that being pumped for cooling must be capable of being cut off if external cooling of the pump is required.
Another problem which arises with canned pumps is that it is difficult to determine if the pump impeller is rotating in the correct direction. If the impeller is not rotating in the correct direction, fluid flow may be 75% of the desired head and damage could result to the pump.
An additional problem of thrust balancing the rotor of canned pumps, is that it is dependent on cooling fluid flow.
Since canned motor driven pumps of the type described use the clearance between the rotor and stator as a variable orifice for controlling fluid flow, they are very sensitive to electrical forces which tend to center the rotor, and make thrust balancing difficult, since the volume of fluid going through the orifice between the rotor and isolated stator and through the bearing varies and can seriosly affect the thrust balance.
The pressure of the pumped fluid is used to operate and control the axial thrust balance of the rotating parts, both electrical hand hydraulic. In a single stage closed impeller of a centrifugal pump, a forward thrust is developed due to the differential pressues developed in the front and rear areas of the impeller. The rear plate of the impeller has the fully developed pressures of genreated forces across its entire diameter. The front of the impeller has a large suction action of 0 or negative pressure, and a lower pressue from the outside diameter of the impeller to the suction inlet. This low pressure is caused by a front wearing ring which permits escapement of fluids from the impeller front face into the suction. It is desirable therefore to hav ea small pressure gradient to reduce or eliminate the pressure leakage back to suction.
The differential of the forces on the rear and the front of the impeller are of great magnitude, which must be neutralized to prevent forward thrust in all other canned motor pumps with the exception of Litzenberg, U.S. Pat. No. 4,065,231. In the prior art, thrust washers are often provided to handle the thrust forces, however, thrust washers do not neutralize the pressure differential and merely transfer the load to the thrust bearing against which the washer bears. The thrust washer method of handling thrust is inefficient since the washer's action is similar to that of a brake shoe, which wastes energy and causes the parts to wear which parts are costly to make and replace.
In addition should the fluid thrust balancing system fail, the thrust washers will get all the load which can quickly wear them out.
The bearings provided in the prior art pumps use a hard mechanical register, are difficult to install and remove, and do not self align, which causes added wear and shorter bearing life that available if the bearings were self aligning to compensate for small variations in the pump components, and applied forces. In my prior U.S. Pat. No. 5,009,578 automatic thrust balancing is provided, which while satisfactory is affected by the flow of cooling fluid. It is therefore desirable to have a system where the fluid flow for cooling, is inependent of the thrust balancing.
The motor driven pumps of my invention are not subject too prior art problems, and provide independently controlled cooling flow, and magnetic thrust balancing.